1. Field of the Invention
The present invention relates to the field of power converters of switched-mode power supply type. The present invention more specifically relates to isolated power supplies, that is, power supplies having no common point between the input voltage (for example, the A.C. supply system) and the regulated D.C. output voltage. The isolation is obtained by means of a transformer having a primary winding associated with a pulse-width modulation controlled switch, and having a secondary winding associated with a diode and with a capacitor providing the output voltage.
2. Discussion of the Related Art
FIG. 1 shows a conventional example of a switched-mode power supply of the type to which the present invention applies. Two input terminals P and N receive an A.C. voltage Vac, for example the mains voltage. Voltage Vac is rectified, for example in a fullwave manner, by means of a diode bridge 1. The A.C. input terminals of bridge 1 are connected to terminals P and N and its rectified output terminals 2 and 3 provide a voltage Vr. Voltage Vr is generally smoothed by means of a capacitor C1 connected between terminals 2 and 3 which form the input terminals of the actual switched-mode power supply.
The converter of FIG. 1 is a so-called flyback converter in which a transformer 4 with inverted phase points has its primary winding 5 connected in series with a switch 6 between terminals 2 and 3. The phase point of winding 5 is connected to a terminal of switch 6, the other terminal of which is connected to terminal 3. Switch 6 is connected in switched mode and at a non-audible high frequency (generally greater than 20 kHz). A secondary winding 7 of transformer 4 is associated with a capacitor C2 across the terminals Sp and Sn of which is provided D.C. output voltage Vout. The phase point of winding 7 is connected to terminal Sp by a diode D1, the cathode of diode D1 being connected to terminal Sp. The other terminal of winding 7 is connected to terminal Sn.
When switch 6 is on, the phase point of winding 7 is at a negative potential. Diode D1 thus is off and a current is stored in primary winding 5. Upon turning off of switch 6, the phase points of windings 5 and 7 both become positive. Diode D1 is forward biased. Capacitor C2 is then charged with the power transferred to secondary winding 7.
Switch 6 (for example, a MOS transistor) is, in the example of FIG. 1, integrated in a circuit 10 with its electronic control circuit. An example of such an integrated circuit, sold by STMicroelectronics Company, is known under trade name VIPER. Circuit VIPER includes an input terminal Vdd intended for receiving a positive power supply, a voltage reference terminal Vss connected to ground, and a terminal FB receiving an error signal. Finally, a terminal 12 is connected to the drain of the integrated N-channel transistor, the source of which is connected to terminal Vss. The gate of transistor 6 is connected at the output of a control circuit 11 (CTRL). Circuit 11 includes a comparator (not shown), a first input of which receives an internal voltage reference and a second input of which is connected, internally, to the positive supply terminal. A VIPER circuit is controlled by a current. The control, that is, the modification of the width of control pulses of switch 6, is performed by, for example, using compensation loop integrated circuit 10, which itself attempts to maintain its supply voltage (Vdd-Vss).
Thus, in an application to a switched-mode converter, terminal Vdd is connected, by a diode D2, to the phase point of an auxiliary winding 8 of transformer 4. The anode of diode D2 is connected to the phase point of the winding. The other terminal of auxiliary winding 8 is connected to reference terminal 3 of the rectified voltage. Auxiliary winding 8 has the function of providing the supply voltage of circuit 10. Terminal FB is connected to the midpoint 13 of a series connection of a zener diode DZ and of a capacitor C3. A capacitor C4 for filtering the local supply voltage is connected between terminal Vdd and terminal 3, the latter being connected to terminal Vss of circuit 10.
In the assembly of FIG. 1, the output voltage is set by the value of the zener diode and the transformation ratio between primary and secondary windings 5 and 7. Auxiliary winding 8, which gives an image of the output voltage, is used, the auxiliary winding being directly in phase with secondary winding 7. The voltage in this winding 8 is thus proportional to the voltage in secondary winding 7.
A disadvantage of the converter of FIG. 1 is that the regulation of output voltage Vout is not very accurate. This disadvantage is illustrated by FIG. 2, which shows the characteristic of output voltage Vout according to the current lout taken by the load connected across terminals Sp and Sn of the converter. It can be considered that, for a nominal voltage Vnom for which the converter is sized, a regulation to more or less 10% of this nominal voltage is obtained for currents ranging between two respectively minimal and nominal values Imin and Inom. Currents Imin and Inom correspond, in practice, to respectively 10% and 100% of the maximum current for which the converter is sized.
When the current surge of the load supplied by the converter is smaller than value Imin, voltage Vout significantly increases as the current decreases. This phenomenon is, among others, due to the fact that noise (voltage peaks) present at the beginning of each demagnetization cycle of auxiliary winding 8 is no longer negligible as compared to the demagnetization period, which is very short. These peaks then strongly influence the value of the voltage across auxiliary winding 8. Capacitor C4 then charges to the maximum value of these peaks.
Between values Imin and Inom, voltage Vout slightly decreases (between +10 and xe2x88x9210% of nominal value Vnom) as the demagnetization period increases. The noise peaks at the beginning of each demagnetization period become more and more negligible.
When the current drawn by the load becomes greater than value Inom, the decrease slope of voltage Vout strongly increases. This is due to the fact that the duty cycle used by the converter is maximum. The output voltage level then cannot be maintained.
More and more often, the low current range (under Imin) is used for power saving reasons (for example, during stand-by periods of the circuits powered by the converter).
To obtain an accurate regulation of output voltage level Vout even for a low current, it is conventionally necessary to provide a regulation of the voltage at the transformer secondary.
FIG. 3 shows an example of a converter implementing such a conventional solution. It shows a transformer 4 having primary and secondary windings 5 and 7 with inverted phase points and having an auxiliary winding 8 providing a supply voltage to a VIPER-type circuit 10. Rectifying bridge 1 and capacitor C1 have not been shown in FIG. 3 but are of course present. As compared to the assembly of FIG. 1, zener diode DZ is replaced with a phototransistor T of an optocoupler 14, the diode D of which conveys a measurement signal coming from the secondary of transformer 4. The anode of diode D is connected, by a resistor R, to D.C. output terminal Sp. The cathode of diode D is connected, by a resistor R1 in series with a zener diode DZ1, to terminal Sn, the anode of diode DZ1 being connected to terminal Sn. When the output voltage reaches the threshold voltage of diode DZ1 in series with the D.C. voltage across diode D of the optocoupler, a current flows through these elements, as well as through the optotransistor. This current flow causes a decrease in the power sent to the secondary by reducing the peak current in switch 6. The gain between the current on terminal FB and this peak current is indeed negative. The more the current is increased on terminal FB, the less power is sent to the secondary.
Other assemblies using a regulation based on a measurement of the voltage at the secondary are known. All these assemblies have in common the use of an additional galvanic isolation component to transmit a regulation order between the secondary and the primary. In the assembly of FIG. 3, said component is optocoupler 14.
The present invention aims at overcoming the disadvantages of known inverted phase point transformer converters.
The present invention aims, in particular, at enabling accurate regulation of the output voltage without it being necessary to use additional galvanic isolation means between the secondary and the primary of the transformer.
To achieve these and other objects, the present invention provides a voltage converter including a circuit for controlling a switch for providing current to a primary winding of a transformer with inverted phase points, a secondary winding of which is associated with a capacitor for providing a regulated D.C. output voltage and an auxiliary winding of which provides a supply voltage of the control circuit, including a means for measuring the average value of the voltage across the auxiliary winding close to the end of its demagnetization periods.
According to an embodiment of the present invention, the measurement means is formed of a resistive and capacitive network forming an averager, the time constant of which is small as compared to the switch control pulse period.
According to an-embodiment of the present invention, said measurement means only receives the voltage of the auxiliary winding during demagnetization periods of the secondary winding.
According to an embodiment of the present invention, the converter includes a means for detecting the demagnetization periods of the auxiliary winding.
According to an embodiment of the present invention, said detection means includes a first circuit providing a two-state signal of detection of the edges of the voltage across the secondary winding, and a second circuit for defining a window in which the detection result of the first circuit is taken into account.
According to an embodiment of the present invention, said window starts with the opening of said switch and ends with the first following zero crossing of the voltage across the auxiliary winding.
According to an embodiment of the present invention, the average voltage across the auxiliary winding is used to control the width of the switch turn-on pulses.
According to an embodiment of the present invention, the switch and its control circuit are integrated in a same circuit.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.